Filtration system having a low profile extruded underdrain

ABSTRACT

An improved underdrain block for an underdrain system supporting a filter media bed in a liquid filtration system is provided. The underdrain blocks are preferably made of an extrudable polymeric material that is extruded in relatively long sections to provide light weight, strong, easily manufactured underdrain systems having a lower profile than prior art underdrain blocks. The underdrain block comprises an upper wall, side walls, a lower wall, at least one lateral member between the upper wall and the lower wall, at least two chambers within the underdrain block, each chamber being defined by the lateral member, a plurality of upper orifices in the upper wall of the underdrain block, and a plurality of internal orifices in the lateral member. The underdrain block is substantially greater in longitudinal length than a longitudinal distance between the upper orifices. In one embodiment, the underdrain block has rails situated on the upper wall for engaging extruded members, which in turn, have receiving recesses for receiving a layer of porous filter media. In another embodiment, the underdrain block further comprises an air nozzle for achieving an improved distribution of air through a filter bed during air backwashing.

The subject patent application is a continuation of U.S. patentapplication Ser. No. 08/935,365 filed on Sep. 22, 1997 which is stillpending.

The present invention relates to filters, and more particularly tounderdrains for liquid filtration systems, particularly water and wastewater filtration systems.

BACKGROUND OF THE INVENTION

Filters and the like that use a filter media bed to remove solids fromliquids are well known. Such systems typically include a liquiddistribution system that collects liquid after it is filtered in oneflow direction and distributes clean liquid through the filter media ina reverse flow direction to effect a cleaning process known asbackwashing.

Water filtration systems of the gravity type are commonly employed forfiltering high volumetric flow rates of liquids, e.g., in municipal andindustrial water treatment and waste water treatment plants. Filtrationsystems generally comprise one or more filters, each employing a bed ofgranular filter media for filtering a liquid as it seeps downwardthrough the filter bed.

Each filtration system generally comprises an open filter basin having afloor and vertical walls surrounding the floor and an underdrainpositioned over the floor. The underdrain defines a perforated falsebottom in the basin for supporting the filter media bed and to provide asystem of fluid passageways for both removing the filtered water fromthe bottom of the filter basin and directing water and/or air into thefilter bed during backwashing.

The filter media bed is generally several feet deep and is typicallycomprised of successive layers of gravel, sand, anthracite, or othergranular filter media. Traditional filter designs use support media suchas multiple gravel layers, beginning with relatively coarse sizes nextto the underdrain's top surface and gradating up to relatively finesizes, are placed on the top surface of the underdrain to prevent thefiner filter media from entering the underdrain and contaminating thefiltered water.

Other filter designs are considered “gravel-less” and use various typesand configurations of porous media to prevent the granular filter mediafrom entering the underdrain. The filter media for both traditional andgravel-less designs consist of one or more layers of sand, anthracite,or other filter media, gradated from coarse on top to fine on bottom,and placed upon the support gravel or porous media.

During operation of the filtration system, the influent, i.e.,unfiltered water, is directed into the filter basin to a depth ofseveral feet above the upper layer of filter media. The influent isallowed to flow downward though the filter media bed. During thisprocess, the suspended materials in the unfiltered water become trappedin the filter media. The water ultimately reaches the bottom of thefilter bed and passes through the perforations in the underdrain system.The water is then collected in a system of fluid passageways within theunderdrain system and is carried out of the filter basin through asuitable conduit or flume.

After the filtration system is operational for an extent of time, theefficiency of the system decreases and it becomes necessary to wash thefilter media bed to remove material trapped therein. Washing of thefilter media is accomplished by utilizing a backwashing process. Thebackwashing process involves pumping pressurized water and/or air in areverse direction into the system of fluid passageways in the underdrainsystem upward through the perforations in the underdrain, and into theoverlying filter media bed. The wash water flowing upwardly through thefilter media bed carries the trapped materials upward from the filterbed. The wash water and the materials entrained or suspended therein arethen collected at the top of the filter basin and carried away.

During the backwashing operation it is desirable to obtain a uniformdistribution of wash water throughout the filter media bed to effectcomplete washing of the entire filter bed. If the wash waterdistribution is uneven so that dead spots occur at certain locationswithin the filter bed, then those portions of the filter bed will beimproperly cleansed, thereby reducing the efficiency of the filter.

The backwashing process must also be performed under carefullycontrolled conditions so as to avoid unduly disturbing or damaging thefilter media bed. For example, the velocity of the wash water must becontrolled at a level below that which would cause the filter media tobecome entrained in the wash water along with the removed materials andcarried away as waste. “Blow holes,” in which explosive bursts of washwater open channels in the filter media at the initiation of thebackwashing cycle, must also be avoided. These blow holes allow influentto pass through the filter media without being filtered and allowfinely-sized filter media to be carried away with the effluent, i.e.,the filtered water.

Several underdrain designs have been developed over the years to addresssome of these potential problems. A common type of underdrain utilizesthe multi-block, modular design in which approximately two- to four-footlong blocks, typically made of either ceramic, cement or plastic, arelaid end to end and disposed next to each other in parallel rows, andthen cemented or grouted in place to form the underdrain.

The interior of a typical block is divided into upper and lowerchambers, i.e., horizontal passageways, separated by a horizontalpartition or lateral, but interconnected by a plurality of orificesformed through the lateral. The multi-block, modular design is commonlyemployed in a filter having a central flume extending below the filterbottom, formed through the concrete supporting structure. The blocksthat are vertically aligned with the flume have at least a portion oftheir bottom walls removed. Thus, the lower chambers are in fluidcommunication with the flume through orifices or cut-outs.

Modular underdrain designs inherently have joints to connect the blocksend-to-end. These joints may be prone to leakage, both external to theblock, or internally, between the various chambers, thus inconsistentlyvarying the hydraulic characteristics of the underdrain.

Most prior art joints designs also induce head loss across the joints,which further adversely affects the hydraulic characteristics of theunderdrain. This head loss increases the pressure drop down the lengthof the underdrain and therefore requires a relatively larger chambercross-sectional area to maintain favorable distribution characteristics.The larger chamber required for this type of underdrain results in alarger overall height and the height of the underdrain directly affectsthe required depth of the filter and associated costs. Thus, it isdesirable to provide an underdrain system which does not have or needjoints.

Prior art underdrains typically utilize porous filter media or capswhich are situated on top of the underdrain system to serve as anadditional filtering mechanism before the influent enters theunderdrain. These caps, preferably porous plates, are usually screwedinto place on the top of the underdrain or held in place with the use ofgaskets. Either way, installation of the porous plates is ratherdifficult and inefficient, making it a time-consuming and costlyprocedure. Maintenance is thus overly burdensome as well, as overperiods of use, the porous plates need to be removed and replaced. Priorart techniques also often result in imperfect seals between theunderdrain and the porous plates. Thus, it is desirable to provide anunderdrain system which provides for improved sealing, a more efficientmanner of installation and reduced maintenance of porous filter media.

In addition, many prior art designs are very heavy structures, whichmakes shipment and installation difficult and costly. Thus, it isdesirable to provide an underdrain system that comprises relativelylight components, making it easier to transport, install, assemble, andmaintain the system.

In general, prior art modular underdrains are very complex, typicallyconsisting of many parts. This complexity, from which many drawbacksresult, creates significantly high production costs as well as highcosts of labor in installing and maintaining the many blocks of modularunderdrain systems. As a primary object of the present, it is thusdesirable to provide an underdrain that comprises less components and isrelatively simple in its design, making it relatively inexpensive tomanufacture, install and maintain.

SUMMARY OF THE INVENTION

An improved underdrain block for an underdrain system supporting afilter media bed in a liquid filtration system is provided. Theunderdrain blocks of the present invention are preferably made of anextrudable polymeric material that is extruded in relatively longsections to provide light weight, strong, easily manufactured underdrainsystems having a lower profile than prior art underdrain blocks.Preferably, the underdrain blocks are made of polyvinyl chloride.

The underdrain block comprises an upper wall, side walls, a lower wall,at least one lateral member between the upper wall and the lower wall,at least two chambers within the underdrain block, each chamber beingdefined by the lateral member, a plurality of upper orifices in theupper wall of the underdrain block, and a plurality of internal orificesin the lateral member. In addition, the underdrain block issubstantially greater in longitudinal length than a longitudinaldistance between the upper orifices.

Preferably, the underdrain block comprises three lateral members withinthe underdrain block comprising two vertical lateral members and onehorizontal lateral member. Preferably, the vertical lateral memberdivides the interior of the underdrain block into three sections ofapproximately equal size and the horizontal lateral member intersectsthe vertical lateral members such that the interiors horizontal lateralmember further divides the interior of the underdrain block into sixchambers. Thus, in the preferred embodiment, there are three upperchamber of approximately equal size located above the horizontal lateralmember and three lower chambers of approximately equal size locatedbelow the horizontal lateral member.

In one embodiment of an underdrain block according to the presentinvention, the underdrain block has rails situated on the upper wall forengaging extruded members. The extruded members have mating recesses forslidably engaging the rails of the underdrain block whereby the extrudedmembers are slidably attached to the underdrain block. The extrudedmembers also have receiving recesses for receiving a layer of porousfilter media such as one or more porous plates. In another embodiment,the rails further comprise support rails on the upper wall of theunderdrain for supporting a layer of filter media and the porous filtermedia.

In another embodiment of the present invention, the underdrain blockfurther comprises side rails located on the side walls of the underdrainblock. The side rails on one side wall are offset from the side rails onthe other side wall of the underdrain block so as to align a pluralityof underdrain blocks side-to-side, whereby the side rails of oneunderdrain block interlock with the side rails of an adjacent underdrainblock.

In another embodiment of the present invention, the underdrain blockfurther comprises an air nozzle for achieving an improved distributionof air through a filter bed during air backwashing. The air nozzlecomprises a pipe having a closed end and an open end, the pipe having avertical slot proximate the closed end and a hole proximate the openend. The air nozzle extends from the lower wall of the underdrain blockto an area proximate the upper wall of said underdrain block, the closedend being located at the lower wall.

In one embodiment, a liquid filtration system includes a porous platehaving upper and lower surfaces. At least one layer of filter media isdisposed above the porous plate. An underdrain is operably associatedwith the porous plate. The underdrain includes first and second membersfor supporting the porous plate. The first and second members each havea lip for retaining the porous plate without the use of screws or bolts.The lips of the first and second members being positioned above theupper surface of the porous plate and extending inwardly toward eachother.

Methods for manufacturing the underdrain block according to the presentinvention are also provided. In addition, methods of filtering aninfluent through an underdrain system of the present invention as wellbackwashing a liquid filtration system and underdrain of the presentinvention are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a preferred embodiment of the lowprofile underdrain according to the present invention.

FIG. 2 is a side view of a wall sleeve in cooperation with an underdrainblock according to the present invention.

FIG. 3 is a cross-sectional view of an alternate preferred embodiment ofthe low profile underdrain according to the present invention.

FIG. 4 is a cross-sectional view of a preferred embodiment of the lowprofile underdrain according to the present invention in cooperationwith porous filter media.

FIG. 4A is an exploded, cross-sectional view of the components of FIG.4.

FIG. 4B is a perspective view of the low profile underdrain of FIG. 4.

FIG. 5 is a cross-sectional view of three air nozzles according to thepresent invention in cooperation with a preferred embodiment of theunderdrain block according to the present invention.

FIG. 6A is a cross-sectional, elevation view of the underdrain blockaccording to the present invention, depicting a plate baffle and aresulting flow pattern.

FIG. 6B is a cross-sectional, elevation view of the underdrain blockaccording to the present invention, depicting multiple tubular bafflesand a resulting flow pattern.

FIG. 7 is a cross-sectional, elevation view of the underdrain blockaccording to the present invention, depicting additional internalorifices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides improved low profile underdrain systems.These systems are used in conjunction with a filter bed system such thataqueous-based fluid is purified in the filter bed system and thefiltrate is collected by the underdrain system. The underdrain systemsof the present invention are preferably made of an extrudable polymericmaterial that is extruded in relatively long sections to provide lightweight, strong, easily manufactured underdrain systems.

The low profile underdrain system of the present invention comprisesrelatively long underdrain blocks. The low profile underdrain blockscomprise blocks of approximately 10, preferably 15, more preferably 20,and even more preferably 25 feet long as opposed to prior art underdrainblocks of approximately 2 to 4 feet long. Underdrain blocks longer than25 feet are also possible.

The blocks of the present invention can be constructed of an extrudablematerial, preferably an extrudable polymeric material. The ability toextrude the polymeric material is advantageous to produce relativelylong underdrain sections in an economical fashion. Preferred extrudablepolymeric materials include polyvinyl chloride (PVC), chlorinatedpolyvinyl chloride (CPVC), high density polyethylene (HDPE), and otherthermoplastic materials. Alternatively, aluminum or steel may be used toextrude the underdrain blocks. PVC, however, is the preferred materialbecause of its light weight, strength, chemical resistance, cost, andgeneral acceptance in the water and waste water industries.

The physical characteristics of the extrudable polymeric material arechosen such that the material can be readily extruded to the desiredshape of the underdrain system. Thus, the underdrain system isconstructed of materials different than those used previously, which aremore suitable for different manufacturing methods, such as short ceramicextrusion and injection molding.

The manufacturing process used to extrude the underdrain system istypically accomplished by heating the material to its melting point,directing it through a die to form the desired cross section, andmaintaining the desired shape until the part has cooled enough to becomedimensionally stable. Due to the intricacies of certain underdraindesigns, care must be taken to control the various processparameters—material temperature, material flow rate, production rate,exit temperature, cooling air flow, etc.—to properly cool the extrudedform to maintain its structural integrity.

Properly directing the extrusion material to different areas of theextrusion die to form the desired part are functions of the materialproperties, the speed at which the part is extruded and the design ofthe part (and thus the die). Parts that have internal webs or otherfeatures that are not accessible from the exterior of the part pose adifficult problem because the cooling of the internal features cannot beeasily controlled. Inadequate cooling can cause distortion of webs orother features, which is detrimental to the structure and aesthetics ofthe part.

Cooling of the part as it comes out of the die can be accomplished bythe use of various combinations of vacuum calibrators and cooling baths.Vacuum calibrators are devices that maintain the cross section of thepart by drawing a vacuum on the part and thereby pulling it against atool that defines the desired shape. Cooling baths circulate a coolingfluid around the part to transfer heat away from the part.

The extrusion process for the present invention requires careful designof the particular underdrain cross section, the die and associated partsdue to the size, weight, and complexity of the part. An underdrain crosssection according to the present invention can typically weighapproximately 6 pounds per foot of extrusion when using PVC. For suchunderdrain designs, multiple combinations of vacuum calibrators andcooling baths can be used in conjunction with careful attention toextrusion rates and quality control procedures to ensure a high qualitypart with acceptable tolerances.

It has been found that not all cross section designs for underdrains canbe extruded to yield a suitable part. For example, the cross section ofa commonly-used underdrain block that is disclosed in U.S. Pat. No.3,110,667, cannot be extruded to the lengths achieved with the presentinvention because of the weight of the material used and the internalwall thickness.

An underdrain design that can be formed in accordance with the extrusionprocess of the present invention resulting in a relatively long block ofuniform cross section is shown in FIG. 1, which depicts across-sectional view of the preferred embodiment of the low profileextruded underdrain. The cross section of the underdrain block 60 has anoverall shape of a rectangle. The underdrain block 60 has an upper wall12, side walls 14, and a lower wall 16. Lateral members or laterals 24define three upper chambers 10 located above three lower chambers 20,each having the shape of a rectangle.

Upper orifices 30 provide passages for water to gain access to the upperchambers 10 of the underdrain system during filtration or for water orair to enter the filter bed from the underdrain 60 during backwashing.Internal orifices 40 provide passages for the water to gain access tothe lower chambers 20 from the upper chambers 10 during filtration orfor water or air to gain access to proceed to upper chambers 10 from thelower chambers 20 during backwashing.

The underdrain block 60 is also equipped with side rails 26 located onthe side walls 14 of the underdrain block 60. The side rails 26 help toalign the underdrain blocks 60 side-to-side in the filter basin. As theside rails 26 on one side wall 14 are offset from the side rails 26 onthe other side wall 14 of the underdrain block 60 so that the rails 26matably align. The side rails 26 provide greater stability of theunderdrain blocks 60 than that obtained by only grouting them in place.

On an alternate embodiment, the side rails 26 on one underdrain block 60will interlock with the side rails 26 of an adjacent underdrain block60. This embodiment will eliminate the requirement for grout betweenadjacent blocks 60.

After extrusion, the long blocks 60 will have no orifices, i.e., neitherupper orifices 30 nor internal orifices 40. Preferably, the upperorifices 30 are provided by drilling holes through the upper wall 12 ofthe underdrain block 60 at the desired pitch, i.e., distance from eachother along the length of the underdrain block 60. Preferably, theinternal orifices 40 are provided by drilling through the lower wall 16of the underdrain block 60. Alternatively, the internal orifices 40 canbe provided by drilling through existing upper orifices 30 if thedesired locations of each should vertically line up.

When the internal orifices 40 are obtained by drilling through the lowerwall 16 of the underdrain block 60, undesired holes 18 will sometimesresult on the lower wall 16. These undesired holes 18 can be pluggedwith a plug or grommet 22 so as not to allow any water or air to pass.Cut-outs will also be provided by drilling or cutting in the lower wall16 of the underdrain block 60 so as to provide a passageway between thelower chambers 20 and a flume.

A flume provides a conduit for the effluent to flow out of the filterbasin during filtration and for pressurized water or air to flow throughthe flume in a reverse direction during backwashing. Alternatively, wallsleeves, which are connected in fluid communication with the end of anunderdrain block 60, e.g., connected to the open end of the lowerchambers 20 of an underdrain block, can be used to serve the samefunction as that of the flume. FIG. 2 depicts a side view of a wallsleeve 96 in cooperation with an underdrain block 60, as the block 60lies on the floor 98 of a filter basin 100. FIG. 2 illustrates the pathof the pressurized water and/or air 88 during backwashing when a wallsleeve 96 is used.

End plates are also needed with the present invention to seal the endsof each chamber 10 and 20 to prevent water or air from bypassing theorifices 30 and 40. End plates are shaped to fit the end of anunderdrain block 60, i.e., the cross section. Preferably, end plates arevacuum molded out of PVC and glued in place with PVC cement.

It should be noted that although certain cross sections cannot beextruded with current extrusion technology, there are manycross-sectional designs that can be made in accordance with the presentinvention. For example, FIG. 3 shows an alternate preferred embodimentof the low profile underdrain according to the present invention. Asshown in FIG. 3, the lateral members 24 within the underdrain block 60define two upper chambers 10 and one lower chamber 20, all of triangularshape.

The upper orifices 30 are spaced at a predetermined pitch along thelength of the underdrain block 60. The size and spacing of the upperorifices 30 are dependent on practical issues regarding hydraulicconsiderations, structural capacity, and manufacturing concerns as wellas on the size and type of filter media placed on top of the underdrain.

Hydraulically, the size and number of the upper orifices 30 must be suchthat the combined area of the orifices provides adequate head loss atthe design flow rate in order to provide the desired flow distribution.Limitations on head loss are the cost and practicality of pumping thebackwash fluid at a high pressure.

The spacing of the upper orifices 30 must be close enough to providegood flow distribution through the filter media. These constraints alsolimit how close the upper orifices 30 may be to each other. It shouldalso be noted that these constraints also limit the size and spacing ofthe internal orifices 40. Additional limitations are required clearancesbetween drills and proper operation of the drill fixture used to createthe orifices 30 and 40. The location of various orifices of a typicalunderdrain is well within the knowledge of those of ordinary skill inthe art and varies for different end uses and applications.

If a support gravel is used on the top of the underdrain, the size ofthe upper orifices 30 must be large enough to preclude media becomingtrapped in the upper orifices 30 and preventing flow. If porous filtermedia such as porous plates are placed on the upper wall 12 of theunderdrain; the orifice size does not have this limitation. Preferably,the underdrain blocks 60 are manufactured at a length substantiallygreater than the pitch for each underdrain block 60.

Another feature of the present invention is that of extrusion rails.FIG. 4 shows the cross-sectional view of a preferred embodiment of theunderdrain block 60 which is equipped to accommodate porous filter mediasuch as porous plates 70. At least one layer of filter media 71 isdisposed above porous plate 70. FIG. 4A shows an exploded,cross-sectional view of the components of FIG. 4. FIG. 4B shows aperspective view of the underdrain block of FIG. 4, illustrating thateach component has a length. In this embodiment, the underdrain block 60has rails 58, shown here as “T”—shaped rails, on the sides of the upperwall 12 of the block 60 substantially above the side walls 14 and hassupport rails 56 on the upper wall 12 of the block 60 directly above thevertical laterals 24 of the underdrain block 60.

Prior to installation of the underdrain blocks 62 in a filter basin,extruded members 90, called F-extrusions, are matably joined to theunderdrain block 60 by sliding the F-extrusions 90 over the T-shapedrails 58. Alternatively, F-extrusions 90 may be glued in place. Matingrecesses 92 of the F-extrusions 90 interlock with the T-shaped rails 58of the underdrain block 60 resulting in a secure fir.

Porous plates 70 are then slid into place in plate-receiving recesses 68on the F-extrusions 90 that are shaped to securely accommodate theporous plates 70. Additional support for the porous plates 70 and filtermedia situated on the top of the porous plate 70 and filter mediasituated on top of the porous plates 70 is also provided by supportrails 56. The underdrain blocks 60 with the porous plates 70 in placeare then installed in the filter basin. Preferably, the F-extrusions 90are extruded with the same material as that of the underdrain blocks 60.In this way, the “T”—shaped rails 58 provide means to secure theF-extrusions 90 in place such that there is a means to secure,removably, the porous plates 70 to the liquid filtration system.

For illustrative purposes, preferably, the overall height of theunderdrain block 62 is approximately 4-8 inches, where the height of theupper chambers 10 is approximately 1-2 inches and the height of thelower chambers 20 is approximately 3-5 inches. Preferably, the overallwidth of the underdrain block 60 is approximately 10-13 inches, wherethe width of each chamber 10 and 20 is approximately 3-4 inches.Preferably, the height of the F-extrusions 90 is approximately 1-2inches, while the thickness of each porous plate 70 is approximately 1inch. Preferably, the diameter of each upper orifice 30 is approximately0.25 inch, while the diameter of each internal orifice 40 isapproximately 0.5 inch. The size of these various members of theunderdrain system is, as is known in the art, dependent on the designflow rate, design head loss and desired distribution characteristics. Itshould also be noted that the size and number of orifices 30 and 40 andchambers 10 and 20 dimensions are interrelated with the above designcharacteristics.

Another feature of the present invention is the incorporation of an airnozzle or air pipe 80. FIG. 5 shows the cross-sectional view of threeair nozzles 80 in cooperation with the underdrain block 60 of thepresent invention. The purpose of the air nozzle 80 is to achieve a moreeven distribution of air through the filter bed during air backwashingthan without the air nozzle 80.

In the preferred embodiment, multiple air nozzles 80 are placed throughthe holes 18 in the lower wall 16 of the underdrain block 60, replacingthe plugs 22, through the lower chamber 20 and into the upper chamber10. In the preferred embodiment, the air nozzle 80 has a vertical slot82 (a lower opening or primary orifice) in the lower portion of the airnozzle 80, situated in the lower chamber 20, and an opening 84 at theopposite end of the air nozzle 80, situated in the upper chamber 10. Asmall hole 86 (an upper opening or metering orifice) is also provided inthe upper portion of the air nozzle 80, situated in the lower chamber20, just below the horizontal lateral 24.

In a typical backwashing procedure, before an air scour is initiated,all of the chambers 10 and 20 of the underdrain are full of water. Thiswater must be displaced in order to allow air to enter the lowerchambers 20, pass into the upper chambers 10 through the internalorifices 40, and exit out of the upper orifices 30. The natural behaviorof the air is to minimally displace the water and take the path of leastresistance, which is typically in the region nearest the air entrancebecause the remaining water impedes the air flow down the length of theunderdrain. This behavior results in grossly uneven distribution of air.By installing air nozzles 80 at discrete intervals and carefully sizingand locating openings 82 and 86 in the air nozzles 80, a means isprovided that allows the air to displace the water down the length ofthe underdrain before air can enter through the air nozzle 80.

As air enters the lower chamber 20, water is displaced by the air andthe water level in the lower chamber 20 drops until the upper opening orupper metering orifice 86 is exposed. At this point, air can enter theair nozzle 80, pass into the upper chamber 10, and exit out of the upperorifices 30. As air flow increases, the pressure drop across the uppermetering orifice 86 increases,causing the water level in the lowerchamber 20 to drop further until the lower opening or primary orifice 82is exposed.

At this point, virtually the entire lower chamber 20 is evacuated ofwater and the air enters the air nozzles 80 through both the meteringand primary orifices 86 and 82. This design allows the air to evenlydisplace the water in the underdrain and gradually and evenly introduceair into the filter media above. This design also allows for awater-only mode in which water enters the air nozzle through the primaryand metering orifices 86 and 82.

As shown in the preferred embodiment in FIG. 5, an air nozzle 80 can beplaced in each of the three sections of the underdrain block 60, i.e.,the middle and both side sections, to ensure the most even distributionof air into the upper chamber 10 and thus, up into the filter bed.Alternatively, only one air nozzle can be used. In this alternativeembodiment, the air nozzle 80 is located only in the middle section andthe air is distributed into the filter bed from the middle section only.This embodiment is less expensive because it only uses one air nozzle asopposed to three. This embodiment also allows for a higher water flowrate through the chambers that do not have air nozzles, but provides aless dense pattern of air distribution.

One benefit from the extruded underdrain of the present invention isthat it provides an underdrain that has a low profile, i.e., it hasconsiderably less height than prior art underdrains. The underdrain ofthe present invention is approximately 4 to 5 inches lower in heightthan prior art two-tier underdrains. This feature helps reduce therequired height of the filter basin and associated costs.

This lower height is a result of a combination of factors. First, headloss is reduced with the extruded design because without joints,associated leakage from joints is eliminated. Thus, the pressure loss ofwater flowing down the length of the underdrain is primarily only lostto friction. Second, because of the internal orifices 40 between thelower chambers 20 and the upper chambers 10, the velocity of waterflowing through the lower chamber 20 decreases along the length of theunderdrain.

The pressure recovery due to the decrease in kinetic energy caused bythe reduction in velocity is greater than the pressure loss due tofriction down the length of the underdrain. This behavior allows thecross-sectional area of the lower chamber 20 to be reduced compared toprior art underdrains in-which the pressure loss due to friction andjoints between underdrain blocks exceeds the pressure recovery due tothe reduction in velocity. With extruded underdrains, the reduction inpressure loss due to friction and the absence of joints and often yieldsvastly different distribution characteristics in comparison to prior artcross-sectional underdrain designs.

The flow rate through an orifice is proportional to the square of thepressure of the water or air upstream of the orifice. The pressuredistribution, and therefore, flow distribution, in both the upperchambers 10 and lower chambers 20 can be affected by installing bafflesin selected areas of the lower chambers 20 or by placing additionalinternal orifices 43 in the horizontal laterals 24 between the upperchambers 10 and lower chambers 20. Baffles may be in the form ofvarious-shaped plates 91 or pipes 93 and may be oriented at angles,perpendicular to the flow, in the direction of the flow, or against theflow, dependent on the degree of pressure regulation desired.

FIG. 6A shows a cross-sectional, elevation view of an underdrain block60 according to the present invention, depicting an angled plate baffle91 and a resulting flow 88 pattern. FIG. 6B is a cross-sectional,elevation view of the underdrain block 60 according to the presentinvention, depicting multiple tubular baffles 93 (pipes) and a resultingflow 88 pattern. Referring to FIG. 6A, the baffle 91 causes a pressuredrop which effectively increases the relative pressure within the localarea 97 of the lower chamber 20 proximate the baffle. This localpressure increase also increases the pressure and flow in the area ofthe upper chamber 10 directly above the baffle.

Similarly, the additional internal orifices 43 in the horizontallaterals 24 separating the chambers 10 and 20 allow additional flow intothe upper chamber 10, i.e., during backwashing. FIG. 7 is across-sectional, elevation view of the underdrain block 60 according tothe present invention, depicting additional internal orifices 43.

When located in the length of the underdrain in areas of low flow, thebaffles 91 and 93 and/or additional internal orifices 43 provide forcorrection of poor flow distribution. When located in strategic areas ofan underdrain, such as in the area of a flume or wall sleeve, thesefeatures help the extruded underdrain achieve better distribution thanhas previously been achieved with prior art underdrains.

The extrusion rails feature of the present invention provides anunderdrain system which utilizes a porous filter media system which doesnot require the use of screws or gaskets. As a result, installation ofporous plates 70 on the extruded underdrain is considerably easier thanprior art porous filter media systems, while sealing of the porousplates 70 is also improved. Thus, the present invention provides anunderdrain system which provides for a more efficient manner ofinstallation and reduced maintenance of porous filter media.

By extruding one relatively long underdrain block 60, the resultingunderdrain system comprises less components than the prior art systems.By using materials such as an extrudable polymeric plastic, theresulting underdrain system is lighter than prior art systems. Prior artunderdrain blocks made of ceramic are also much more susceptible todamage such as chipping, and their weight contributes to this drawback.Fewer components and lighter components make the present inventioneasier to ship, install, assemble and maintain.

By extruding one relatively long underdrain block 60, the resultingunderdrain system is considerably less complex than the prior artsystems. For example, no joints are necessary to connect the longerblocks 60, end-to-end or side-to-side. This design significantly reducesproduction costs with respect to manufacturing and installation.Maintenance costs are also reduced because there are less componentsthat can malfunction.

It is to be understood that even though numerous characteristics andadvantages of the present invention have been set forth in the foregoingdescription, together with details of the structure and function of theinvention, the disclosure is illustrative only, and changes may be madein detail, especially in matters of shape, size and arrangement of partswithin the principles of the invention to the full extent indicated bythe broad general meaning of the terms in which the appended claims areexpressed.

What is claimed is:
 1. An improved liquid filtration system comprising:(a) a filter bed comprising: a layer of porous filter media; and atleast one layer of filter media situated on top of said layer of porousfilter media; (b) an underdrain situated underneath said layer of porousfilter media, said underdrain having an upper wall and rails situated onsaid upper wall for engaging extruded members; and (c) a plurality ofextruded members having mating recesses for slidably engaging said railsof the underdrain whereby the extruded members are slidably attached tothe underdrain.
 2. The liquid filtration system of claim 1, wherein saidplurality of extruded members further comprise receiving recesses forreceiving said layer of porous filter media.
 3. The liquid filtrationsystem of claim 2, wherein said layer of porous filter media iscomprised of one or more porous plates.
 4. The liquid filtration systemof claim 2, wherein said rails further comprise support rails on saidupper wall of said underdrain for supporting said layer of filter mediaand said layer of porous filter media.
 5. The liquid filtration systemof claim 1, wherein said underdrain further comprises side walls andside rails located on said side walls of the underdrain block, said siderails on one side wall being offset from the side rails on the otherside wall of the underdrain block, said side rails are provided to aligna plurality of underdrain blocks side-to-side.
 6. The liquid filtrationsystem of claim 5, whereby said side rails of one underdrain blockinterlock with said side rails of an adjacent underdrain block.
 7. Theliquid filtration system of claim 1, wherein said underdrain furthercomprises a lower wall and said liquid filtration system furthercomprises: an air nozzle comprising a pipe having a closed end and anopen end, said pipe having a first opening proximate the closed end,whereby said air nozzle extends from the lower wall of the underdrainblock to an area proximate the upper wall of said underdrain block, saidclosed end being located at the lower wall.
 8. The liquid filtrationsystem of claim 7, wherein said pipe further comprises a second openingproximate the open end.
 9. A liquid filtration system comprising: (a) aporous plate having upper and lower surfaces; (b) at least one layer offilter media disposed above said porous plate: (c) an underdrainincluding first and second members for supporting said porous plate,said first and second members each having a lip for retaining saidporous plate without the use of screws or bolts, said lips of said firstand second members being positioned above said upper surface of saidporous plate and extending inwardly toward each other.
 10. A liquidfiltration system as recited in claim 9, wherein: (a) said underdrainincludes an underdrain block having an upper wall and first and secondrails situated on said upper wall; and, (b) said first member isattached to said first rail, said second member is attached to saidsecond rail.
 11. A liquid filtration system as recited in claim 9,wherein: (a) said underdrain includes an underdrain block having anupper wall and first and second rails situated on said upper wall; and,(b) said first member is slidably attached to said first rail, saidsecond member is slidably attached to said second rail.
 12. An improvedliquid filtration system comprising: (a) filter media; (b) an underdrainsituated underneath said filter media, said underdrain having an upperwall and first and second rails; (c) first and second extruded membershaving mating recesses for slidably engaging said first and secondrails, respectively, of said underdrain whereby said first and secondextruded members are slidably attached to said underdrain.